import qualified Data.ByteString.Lazy.Char8 as LC
import Data.Maybe (fromJust)

type Equation a = (a, a)

allTheSame :: (Eq a) => [a] -> Bool
allTheSame xs = all (== head xs) $ tail xs

solve :: Integral a => [Equation a] -> a
solve xs
    | allTheSame anss && ans1 > 0 = ans1
    | otherwise = -1
    where
        anss = fmap (\(x, y) -> y - x) xs
        ans1 = head anss

tuplify2 :: [a] -> (a, a)
tuplify2 [x, y] = (x, y)
tuplify2 _ = error "the length of the list must be two"

unsafeReadInt :: LC.ByteString -> Int
unsafeReadInt = fst . fromJust . LC.readInt

main :: IO ()
main = getLine >> LC.getContents >>= print . solve .
    fmap (tuplify2 . fmap unsafeReadInt . LC.words) . LC.lines